Microelectronic packages having an array of resilient leads and methods therefor

ABSTRACT

A method of making a microelectronic package having an array of resilient leads includes providing a first element having a plurality of conductive leads at a first surface thereof, the conductive leads having terminal ends permanently attached to the first element and tip ends remote from the terminal ends, the tip ends being movable relative to the terminal ends. A second element having a plurality of contacts on a first surface thereof is then juxtaposed with the first surface of the first element, and the tip ends of the conductive leads are connected with the contacts of the second microelectronic element. The first and second elements are then moved away from one another so as to vertically extend the conductive leads between the first and second elements. After the moving step, a layer of a spring-like conductive material is formed over the conductive leads to form composite leads. The layer of a spring-like material desirably has greater yield strength than the conductive leads, thereby enhancing the resiliency of the composite lead structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims benefit of U.S. Provisional ApplicationNo. 60/236,395, filed Sep. 29, 2000, the disclosure of which is herebyincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to microelectronic packages having leadsor traces and specifically relates to microelectronic packages havingarrays of resilient leads and methods of making such microelectronicpackages.

BACKGROUND OF THE INVENTION

Complex microelectronic devices such as semiconductor chips typicallyrequire numerous connections to other electronic components. Forexample, a complex device including a semiconductor chip may requirehundreds of electrical connections between the chip and one or moreexternal devices. These electrical connections may be made using severalalternative methods, including wire bonding, tape automated bonding andflip-chip bonding. Each of these techniques presents various problemsincluding difficulty in testing the chip after bonding, long leadlengths, large areas occupied by the chip on a microelectronic assembly,and fatigue of the connections due to changes in size of the chip andthe substrate during thermal expansion and contraction.

In many microelectronic devices, it is desirable to provide anelectrical connection between components that can accommodate relativemovement between the components. For example, where a semiconductor chipis mounted to a circuit board, thermal expansion and contraction of thechip and circuit board can cause the contacts on the chip to moverelative to contacts on the circuit board. This movement can occurduring operation of the device and can also occur during manufacturingoperations (e.g. when soldering the chip to the circuit board).

One structure that has been used to successfully address these problemsis commonly referred to as an “interposer” or “chip carrier”, such asthat shown in certain preferred embodiments of commonly assigned U.S.Pat. Nos. 5,148,265, 5,148,266 and 5,455,390, the disclosures of whichare hereby incorporated by reference herein. Interposers typicallyinclude a flexible, sheet-like element having a plurality of terminalsdisposed thereon, and including flexible leads used to connect theterminals with contacts on a microelectronic element, such as asemiconductor chip or wafer. The flexible leads permit thermal expansionof the various components without inducing stresses in the connection.The terminals of the interposer may then be used to test the assembly,and/or permanently attach the assembly to another microelectronicelement.

A compliant layer may be disposed between a microelectronic element andthe interposer. The compliant layer typically encapsulates the leadsconnecting the interposer and microelectronic element and facilitatesconnection of the terminals to a test device and/or to the finalelectronic assembly by compensating for variations in component flatnessand terminal heights.

As illustrated in certain preferred embodiments of commonly assignedU.S. Pat. No. 5,518,964 (“the '964 patent”), the disclosure of which ishereby incorporated by reference herein, an array of moveable electricalconnections between two microelectronic elements, such as asemiconductor chip and a substrate, can be provided by first connectingleads between the microelectronic elements and then moving the elementsaway from one another through a predetermined displacement so as to bendthe leads. One of the microelectronic elements may be a connectioncomponent including a dielectric body having leads extending along asurface of the dielectric body. The leads may have first endspermanently attached to the dielectric body and second ends releasablyattached to the dielectric body. The dielectric body, with the leadsthereon, may be juxtaposed with a semiconductor chip having contacts andthe second releasable ends of the leads may be bonded to the contacts onthe chip. Following bonding, the dielectric body and chip are moved awayfrom one another, thereby bending the leads toward a verticallyextensive disposition. During or after movement, a curable material suchas a liquid composition may be introduced between the elements. Thecurable material may then be cured, such as by using heat, to form acompliant dielectric layer surrounding the leads. The resultingsemiconductor chip package has terminals on the dielectric body orconnection component which are electrically connected to the contacts onthe chip, but which can move relative to the chip so as to compensatefor thermal effects. For example, the semiconductor chip package may bemounted to a circuit board by solder-bonding the terminals to conductivepads on the circuit board. Relative movement between the circuit boardand the chip due to thermal effects is allowed by the moveableinterconnection provided by the leads and the compliant layer.

In other embodiments of the '964 patent, the package-forming process canbe conducted on a wafer scale, so that all of the semiconductor chips ina wafer may be connected to connection components in a single step. Theresulting wafer package is then severed so as to provide individualunits, each including one or more of the chips and a portion of thedielectric body. The above-described leads may be formed on the chip orwafer, rather than on the dielectric body. In further embodiments of the'964 patent, a dielectric body having terminals and leads is connectedto terminal structures on a temporary sheet. The temporary sheet anddielectric body are moved away from one another so as to verticallyextend the leads, and a curable liquid material is introduced around theleads and cured so as to form a compliant layer between the temporarysheet and the dielectric body. The temporary sheet is then removed,leaving the tip ends of the terminal structures projecting from asurface of the compliant layer. Such a component, commonly referred toas a connection component, may be used between two other components. Forexample, the terminal structures may be engaged with a semiconductorchip and the terminals engaged with a circuit panel or othermicroelectronic component.

In certain preferred embodiments of commonly assigned U.S. Pat. No.6,117,694, the disclosure of which is hereby incorporated herein byreference, a microelectronic component, such as a connector or apackaged semiconductor device, is made by connecting multiple leadsbetween a pair of elements and moving the elements away from one anotherso as to bend the leads toward a vertically extensive disposition. Oneof the elements may include a temporary support that may be removedafter bending the leads

After the leads interconnect the microelectronic elements, anencapsulant, such as a flowable, curable dielectric material, may beinjected between the microelectronic elements. The encapsulant may beinjected between the microelectronic elements immediately after bonding,whereby the force of the pressurized encapsulant acting on the elementsseparates them and bends the leads, forming a compliant leadconfiguration. Alternatively, the leads may be formed before injectingthe encapsulant by retaining the elements against moveable platens byvacuum, and moving the platens with respect to each other, bending andforming the leads. The encapsulant is then injected while the dielectricsheet and the wafer are in their displaced positions.

After the flowable, curable dielectric material has been cured, themicroelectronic assembly may be removed from the fixture, trimmed andtested. The fixture may then be reused to perform the above operationson the next microelectronic assembly.

Despite these and other advances in the art, still further improvementswould be desirable.

SUMMARY OF THE INVENTION

In accordance with certain preferred embodiments of the presentinvention, a method of making a microelectronic package having an arrayof resilient leads includes providing a first element having a pluralityof conductive leads on a first surface thereof. The conductive leadspreferably have terminals ends permanently attached to the first elementand tip ends remote from the terminal ends, the tip ends of theconductive leads being movable relative to the terminal ends. The methodpreferably includes providing a second element having a plurality ofcontacts on a first surface thereof and juxtaposing the first surface ofthe second element with the first surface of the first element. The tipends of the conductive leads may then be connected with the contacts ofthe second microelectronic element. The first and second microelectronicelements may then be moved away from one another so as to verticallyextend the conductive lead between the first and second microelectronicelements. After the moving step, a layer of a spring-like material maythen be formed over the conductive leads. The layer of a spring-likematerial preferably has greater yield strength than the conductiveleads. Although the present invention is not limited by any particulartheory of operation, it is believed that providing a layer of aspring-like material having greater yield strength then the yieldstrength of the conductive leads produces a highly resilient compositelead able to withstand substantial flexing and bending. As used herein,the term “composite lead” means a conductive lead or trace having a coremade of a first conductive material that is coated by a shell of asecond conductive material.

The conductive leads may be made of a material selected from the groupconsisting of aluminum, gold, copper, tin, and their alloys andcombinations thereof. The layer of a spring-like material formed overthe conductive leads is preferably selected from the group consisting ofnickel, copper, cobalt, iron, gold, silver, platinum, noble metals,semi-noble metals, tungsten, molybdenum, tin, leads, bismuth, indium,their alloys, and combinations thereof.

In certain preferred embodiments, the method includes depositing acurable liquid encapsulant between the first and second microelectronicelements and around the vertically extended composite leads. Inpreferred embodiments, the curable liquid encapsulant is selected fromthe group consisting of materials that are curable to elastomers andadhesives. The preferred elastomers and adhesives are selected from thegroup consisting of silicones and epoxies. In highly preferredembodiments, the curable liquid encapsulant is a composition which iscurable to a silicone elastomer. After the curable liquid encapsulant isdeposited, the encapsulant may be cured to provide a compliant layerbetween the first and second microelectronic elements and around thevertically extended leads. In preferred embodiments, the terminals areaccessible at the second surface of the second microelectronic element.Conductive elements, such as solder balls, may then be attached to theterminals ends of the leads. The conductive elements may be fusiblemasses of conductive metal, such as tin/lead solder balls.

In preferred embodiments, the first element and/or the second element isa microelectronic element. In preferred embodiments, the microelectronicelements are selected from the group consisting of a semiconductorchips, semiconductor wafers, packaged semiconductor chips or wafers,dielectric sheets, flexible substrates, flexible circuitized substrates,printed circuit boards, and sacrificial layers. In more preferredembodiments, the first and second microelectronic elements are selectedfrom the group consisting of semiconductor chips, semiconductor wafers,and flexible substrates. In particularly preferred embodiments, thefirst microelectronic element is a chip and the second microelectronicelement is a flexible substrate. In certain embodiments, at least one ofthe microelectronic elements may be a sacrificial layer. The sacrificiallayer may be removed during one step of an assembly process so as toexpose either the terminal ends or the tip ends of the conductive leads.In an alternative embodiment, the sacrificial layer may be conductiveand the terminal ends or the tips ends of the conductive leads may beformed and/or exposed by removing a portion of the conductivesacrificial layer.

In other embodiments, a method of making a microelectronic packagehaving a plurality of resilient leads includes providing a firstmicroelectronic element having conductive leads extending along a firstsurface thereof, the conductive leads having terminal ends permanentlyattached to the first microelectronic element and tip ends releasablysecured to the first microelectronic element. A second microelectronicelement having contacts on a first surface thereof, may then bejuxtaposed with the first surface of the first microelectronic elementand the tip ends of the conductive leads may be connected with thecontacts of the second microelectronic element. The first and secondmicroelectronic elements may then be moved away from one another so asto vertically extend the conductive leads between the first and secondmicroelectronic elements. In certain preferred embodiments, the tip endsof the conductive leads may be releasably secured to the firstmicroelectronic element. After the moving step, a layer of a spring-likematerial may be formed over the conductive leads. The layer of aspring-like material may be formed by plating a conductive metal overthe conductive leads. The conductive metal plated over the conductiveleads preferably has a higher yield strength than the conductive leads.A curable liquid encapsulant may then be disposed between the first andsecond microelectronic elements and around the conductive leads. Thecurable liquid encapsulant may be cured to form a compliant layerbetween the microelectronic elements and around the leads.

In still other preferred embodiments of the present invention, a methodof making a microelectronic package includes providing a firstmicroelectronic element having a first surface with a plurality ofconductive leads formed thereon, each lead having a first endpermanently attached to the first microelectronic element and a secondend movable away from the first microelectronic element. A secondmicroelectronic element having conductive pads accessible at a firstsurface thereof may then be provided and the first surface of the firstmicroelectronic element may be juxtaposed with the first surface of thesecond microelectronic element. The method may also include attachingthe second ends of the conductive leads with the conductive pads orcontacts of the second microelectronic element. After the attachingstep, the first and second microelectronic elements may be moved awayfrom one another so as to vertically extend the conductive leads. Afterthe moving step, a layer of a conductive metal may be formed over theconductive leads, the layer of a conductive metal having a greater yieldstrength then the conductive leads.

In certain preferred embodiments, the first microelectronic element is aflexible substrate, such as a flexible dielectric sheet, and the secondmicroelectronic element is a semiconductor chip or a semiconductorwafer. In other preferred embodiments, the first microelectronic elementis a semiconductor chip or wafer and the second microelectronic elementis a flexible substrate such as a flexible dielectric sheet. If thefirst microelectronic element is a wafer, the method may also includesevering the semiconductor wafer and the flexible dielectric sheet toprovide a plurality of semiconductor packages, whereby eachsemiconductor package includes at least one semiconductor chip connectedto a portion of the flexible dielectric sheet.

In yet other preferred embodiments of the present invention, a method ofmaking semiconductor packages having resilient leads includes providinga semiconductor chip or wafer having a plurality of contacts on a firstsurface thereof, and providing a flexible dielectric sheet having aplurality of conductive leads over a first surface thereof, whereby eachlead has a terminal end permanently attached to the flexible dielectricsheet and a tip end movable away from the first surface of thedielectric sheet. The tip ends of the leads may then be electricallyinterconnected with the contacts of the wafer. If a wafer is used, thenthe wafer and the dielectric sheet may then be moved away from oneanother in a controlled fashion so as to vertically extend the leads.After the wafer and dielectric sheet are moved away from one another, alayer of a spring-like material may be formed over the outer surface ofthe conductive leads to form composite leads. The spring-like materialdesirable has a greater yield strength than the yield strength of theconductive leads. A layer of a compliant material may then be disposedbetween the wafer and the dielectric sheet and around the compositeleads. The wafer and the dielectric sheet may then be severed so as toprovide a plurality of semiconductor packages, each semiconductorpackage including at least one semiconductor chip and a portion of thedielectric sheet.

In further preferred embodiments of the present invention, a method ofmaking a microelectronic element includes providing a dielectric sheethaving a plurality of conductive leads overlying the first surface ofthe sheet and a plurality of terminals accessible at a second surface ofthe sheet, whereby each lead has a first end permanently attached to oneof the terminals and a second end movable away from the first surface ofthe dielectric sheet. The method includes providing a fixture having afirst surface and a plurality of contacts accessible at the firstsurface of the fixture, juxtaposing the first surface of the fixturewith the first surface of the dielectric sheet and attaching the secondends of leads with the contacts of the fixture. After the attachingstep, the fixture and the dielectric sheet may be moved away from oneanother so as to vertically extend the leads. A layer of a conductivespring-like material may then be formed over the exterior surface of theconductive leads to form composite leads, and a layer of a curableliquid encapsulant may be provided between the fixture and thedielectric sheet and around the composite leads. The encapsulant maythen be cured to form a compliant layer. After the curing step, thefixture may be removed so as to expose the contacts at a top surface ofthe package.

In yet other preferred embodiments of the present invention, aconnection component includes a flexible substrate having a top surfaceand a bottom surface, and a layer of a compliant dielectric materialoverlying the top surface of the substrate, the compliant materialhaving a top surface remote from the substrate. The connection componentincludes an array of flexible, conductive leads having first endsattached to terminals accessible at the second surface of the saidsubstrate and second ends adjacent the top surface of the compliantlayer. Each lead preferably includes a core made of a first conductivematerial that is surrounded by a layer of a second conductive material.Such leads are similar to the composite leads described above. Thesecond conductive material of the lead preferably has a greater yieldstrength than the first conductive material. The second ends of theleads may be accessible at the tope surface of the compliant layer. Theconnection component may also include contacts attached to the secondends of the leads, the contacts being accessible at the top surface ofthe compliant layer.

These and other preferred embodiments of the present invention will beset forth in further detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1G show a method of making a microelectronic package having anarray of resilient leads, in accordance with one preferred embodiment ofthe present invention.

FIGS. 2A-2I show a method of making microelectronic packages havingarrays of resilient leads, in accordance with further preferredembodiments of the present invention.

FIGS. 3A-3H show a method of making a compliant connection componenthaving an array of resilient leads, in accordance with still furtherpreferred embodiments of the present invention.

FIG. 4 shows leads having tip ends releasably attached to a substrate,in accordance with preferred embodiments of the present invention.

FIG. 5 shows leads having tip ends releasable attached to a substrate,in accordance with other preferred embodiments of the present invention.

FIG. 6 shows a variety of leads formed atop a substrate, in accordancewith still other preferred embodiments of the present invention.

FIG. 7 shows leads shown and restraining straps formed atop a substrate,in accordance with further preferred embodiments of the presentinvention.

FIG. 8 shows leads formed atop a substrate, in accordance with stillfurther preferred embodiments of the present invention.

FIG. 9 shows leads formed atop a substrate in accordance with yetfurther preferred embodiments of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1A, a substrate 20 includes a first surface 22 and asecond surface 24 remote therefrom. Although substrate 20 may be rigid,semi-rigid or flexible, in preferred embodiments substrate 20 isflexible, such as a flexible dielectric sheet. The flexible substrate 20includes a plurality of flexible conductive leads 26 formed on the firstsurface 22 thereof. The flexible conductive leads 26 may be made from awide variety of materials, including gold, aluminum, copper, theiralloys, and combinations thereof. Each conductive lead 26 desirableincludes a terminal end 28 permanently secured to flexible substrate 20and a tip end 30 remote from the terminal end. As will be described inmore detail below, the tip ends 30 of the leads 26 are preferablyreleasably attached to and movable away from the top surface 22 offlexible substrate 20. The terminal end 28 of each lead 26 is preferablyaligned with an opening 32 extending between the first and secondsurfaces 22, 24 of flexible substrate 20. In preferred embodiments, theflexible substrate 20 is comprised of a sheet of a dielectric material,more preferably of a sheet of a polymeric dielectric material. Inparticularly preferred embodiments, flexible substrate 20 is comprisedof a sheet of polyamide.

As will be described in more detail below, the flexible substrate 20 ispreferably assembled to another microelectronic element. Referring toFIG. 1B, one such microelectronic element is a semiconductor wafer 34having a contact bearing face 36 including a plurality of contacts 38formed on the contact bearing face, and a rear face 40 remote fromcontact bearing face 36. The plurality of contacts 38 are preferablypositioned in an array over contact bearing face 36 of wafer 34. Whensemiconductor wafer 34 is positioned over the first surface 22 offlexible substrate 20, the contacts 38 are preferably placed insubstantial alignment with the tip ends 30 of conductive leads 26.

Referring to FIG. 1C, the contact bearing face 36 of semiconductor wafer34 is then juxtaposed with the first surface 22 of flexible substrate 20so that contacts 38 are in substantial alignment with tip ends 30 ofconductive leads 26. A conductive paste (not shown) may be applied tothe tip ends 30 of conductive leads 26 in order to temporarily attachcontacts 38 to the tip ends 30. The leads 26 may be permanently attachedto contacts 38 by bonding the tip ends 30 of leads 26 to contacts 38.

The tip ends 30 of the conductive leads 26 are preferably peelable orreleasable from the first surface 22 of flexible substrate 20. Adhesionbetween the flexible substrate 20 and the tip ends 30 of leads 26 may bereduced by using the methods disclosed in commonly assigned U.S. Pat.No. 5,763,941; and U.S. patent applications Ser. No. 09/020,750;09/200,100; 09/225,669; 09/566,273; 09/577,474; 09/317,675; and09/757,968, the disclosures of which are hereby incorporated byreference herein. For example, prior to forming the conductive leads 26atop the flexible substrate 20, an adhesion reducing substance such assilicone may be provided over the first surface 22 of substrate 20 forreducing the level of adhesion between tip ends 30 and substrate 20. Inthe particular embodiment shown in FIGS. 1A-1C, the tip ends 30 of leads26 are commonly referred to as being releasable and the terminal ends 28of leads 26 are commonly referred to as being fixed. In embodimentswhere the substrate is made of a polymeric material, there may be noneed to take affirmative steps to enhance peelability between leads 26and flexible substrate 20 because poor adhesion generally resultsbetween leads 26 and polymeric layers.

Referring to FIG. 1D, after leads 26 are attached to contacts 38, thesemiconductor wafer 34 and the flexible substrate 20 are moved away fromone another through a controlled displacement using platens 40, 42 asdisclosed in commonly assigned U.S. Pat. No. 5,801,441, the disclosureof which is hereby incorporated by reference herein. A vacuum ispreferably applied through platen 40 for firmly holding semiconductorwafer 34 and through platen 42 for firmly holding flexible substrate 20.One or both of the platens are moved so that semiconductor wafer 34moves vertically away from flexible substrate 20 in the directionindicated by arrow V₁. At the same time, platen 40 and semiconductorwafer 34 may be moved horizontally relative to platen 42 and flexiblesubstrate 20 in a horizontal direction indicated H₁. Stated another way,flexible substrate 20 may also be moved in a horizontal direction suchthat the horizontal component of motion of the flexible substrate 20 isin a second direction H₂, opposite the first horizontal direction H₁.Thus, the semiconductor wafer 34 and the tip ends 30 of the leads 26move, relative to the flexible substrate 20 and the terminal ends 28 ofleads 26, along the direction indicated by A₁. The vertical movementtypically is about 100-500 microns, and the horizontal movement istypically approximately equal to the vertical movement. During thecontrolled movement, the tip ends 30 of the leads 26 peel away from thefirst surface 22 of the flexible substrate 20. The terminal ends 28 ofthe lead 26 remain fixed to the flexible substrate 20. During movementof the semiconductor wafer 34 and the flexible substrate 20 away fromone another, the leads 26 deform and/or bend in a vertical directionaway from the flexible substrate 20 and the terminal ends 28 thereof.

When the wafer 34 is moved in the direction indicated by A₁, the neteffect of the relative movement of the semiconductor wafer 34 and theflexible substrate 20 is to move the tip ends 30 of conductive lead 26horizontally towards and vertically away from the terminal ends 28 ofthe same leads, thus forming each flexible lead 26 into a verticallyextensive, curved structure as illustrated in FIG. 1D. Such a leadstructure is able to flex and bend so as to compensate for movement ofwafer 34 and substrate 20 relative to one another. In other embodiments,the movement of the semiconductor wafer 34 and flexible substrate 20 maynot include a horizontal component, but only a vertical component. Inthese embodiments, the vertical movement will serve to partiallystraighten the leads 26. In preferred embodiments, some slack is left inthe vertically extended leads 26 so as to allow for subsequent movementof wafer 34 and substrate 20 relative to one another.

Referring to FIG. 1E, after the semiconductor wafer 34 and flexiblesubstrate 20 have been moved away from one another so as to verticallyextend leads 26, a spring-like material preferably is formed over theouter surface of leads 26. The layer of spring-like material 44preferably has substantially higher yield strength than the materialcomprising the flexible lead 26. In preferred embodiments, thespring-like material 44 is selected from the group consisting of nickel,copper, cobalt, iron, tin, lead, bismuth, indium, gold, silver,platinum, tungsten, molybdenum, semi-noble metals, their alloys, andcombinations thereof. The layer of spring-like material may beelectroplated or may be formed by sputtering, chemical vapor depositionor combinations of any of the above methods. Although the presentinvention is not limited by any particular theory of operation, it isbelieved that the formation a layer of a spring-like material oververtically extended conductive leads 26 will substantially enhance theresiliency of the composite leads 46.

Referring to FIG. 1F, after forming the layer of a spring-like material44 around leads 26, an encapsulant 48 such as a curable liquid materialis preferably introduced between semiconductor wafer 34 and flexiblesubstrate 20 and around composite leads 46. Preferred methods fordisposing an encapsulant layer between microelectronic elements aredisclosed in certain preferred embodiments of the above-mentioned U.S.Pat. No. 5,801,441. The encapsulant preferably has a low viscosity andis introduced in an uncured state. The encapsulant 48 preferably wets tothe semiconductor wafer 34 and flexible substrate 20, effectively fillsa gap therebetween and penetrates between composite leads 46. Theencapsulant may be rigid or compliant. In preferred embodiments, theencapsulant 48 is selected so that it will form a compliant material,such as a gel or an elastomer, upon being cured. Preferred encapsulantsinclude silicones and epoxies, with silicone elastomers and flexiblizedepoxies being particularly preferred. In some embodiments, theencapsulant around the composite leads 46 is rigid and the remainder ofthe encapsulant between semiconductor chip or wafer 34 and flexiblesubstrate 20 is compliant. In still other embodiments, the encapsulantaround the composite leads 46 is compliant and the remainder of theencapsulant 48 between semiconductor wafer 34 and flexible substrate 20is rigid.

In its liquid state, the encapsulant 48 may be injected under pressure.The encapsulant may also be injected without external pressure andallowed to fill the gap between semiconductor wafer 34 and flexiblesubstrate 20 only by capillary action. After being disposed betweensemiconductor wafer 34 and flexible substrate 20 and around compositeleads 46, the encapsulant is cured in placed. Depending upon theformulation of the encapsulant, such curing may take place spontaneouslyat room temperature or else may require exposure to energy, such as heator radiant energy.

Referring to FIG. 1G, after encapsulant layer 48 has been cured toprovide a compliant or resilient layer between semiconductor wafer 34and flexible substrate 20, conductive elements 50 may by attached to theterminal ends 28 of composite leads 46. The conductive elements 50 arepreferably tin/lead solder balls that extend through the openings 32 inthe flexible substrate 20. The conductive elements 50 may be reflowed soas to permanently attach the conductive elements 50 to terminal ends 28of composite leads 46. Upon being reflowed, the conductive elements 50preferably form an intermetallic bond with the terminal ends 28 of theleads 26. Surface tension may also result in the reflowed conductiveelements 50 having a substantially spherical shape. In other preferredembodiments, the conductive elements 50 may include material such asgold and platinum.

Referring to FIG. 2A, in accordance with further preferred embodimentsof the present invention a first microelectronic component 134, such asa semiconductor wafer, has a first surface 136 and a second surface 140remote therefrom. The first surface 136 of semiconductor wafer 134 has aplurality of conductive traces or leads 126 formed thereon. Eachconductive lead 126 includes a first end 130 releasably attached tofirst face 136 and a second end 128 permanently attached to wafer 134.

Referring to FIG. 2B, the front face 136 of wafer 134 is preferablyjuxtaposed with a flexible substrate 120. In a particular preferredembodiment shown in FIG. 2B, the flexible substrate 120 is a two-metaltape having a first surface 122 and a second surface 124 remotetherefrom. The flexible tape 120 includes a series of vias 132 extendingbetween the first and second surfaces 122, 124 thereof. Each via 132preferably has a layer of a conductive metal 152 deposited therein. Eachlayer of conductive material 152 deposited in vias 132 preferablyincludes a flange region 154 that extends outwardly from the via 132along the second surface 124 of substrate 120.

Referring to FIG. 2C, the first face 136 of semiconductor wafer 134 isjuxtaposed with top surface 122 of flexible tape 120. The releasablefirst ends 130 of conductive leads 126 are preferably placed insubstantial alignment with the conductive metal 152 deposited in thevias 132. A portion 154 of metal layer 152 is preferably accessible atthe top surface 122 of flexible tape 120. The wafer 134 is moved towardthe top surface 122 of flexible tape 120 until the conductive leads 126contact the deposited metal 152 accessible at the first surface 122 offlexible tape 120. Immediately before the first ends 130 of leads 126contact the metal portion 154, a conductive paste or adhesive 156 may beapplied to the releasable ends 130 of leads 126. The conductive adhesiveallows the leads to be attached to the metal portion 154. FIG. 2D, showsthe releasable ends 130 of leads 126 attached to metal portion 154 ofthe metalized vias 132.

Referring to FIG. 2E, semiconductor wafer 134 and flexible tape 120 arethen moved away from one another in a controlled manner using platens140 and 142 as described above in reference to FIG. 1D. As semiconductorwafer 134 and flexible tape 120 move away from one another, conductiveleads 126 are vertically extended.

Referring to FIG. 2F, a layer of a spring-like material, such as nickel,is then formed over the exterior surface of each conductive lead 126. Asmentioned above, the layer of spring-like material 144 preferably has arelatively higher yield strength than the yield strength of theconductive leads 126. Together, the conductive leads 126 with the layerof a spring-like material formed thereon comprise composite leads 146.

Referring to FIG. 2G, after composite leads 146 have been formed, acurable encapsulant may then be disposed between the front face 136 ofsemiconductor wafer 134 and the first surface 122 of flexible tape 120.As mentioned above, the curable encapsulant is preferably disposedbetween the wafer and tape while the curable encapsulant is in a liquidform. The encapsulant may then be cured in situ by applying energy orexposing the encapsulant to atmosphere. The cured encapsulant layer ispreferably compliant so as to compensate for thermal expansion andcontraction of the wafer 134 and substrate 120 during assembly andoperation of the microelectronic package.

Referring to FIG. 2H, conductive elements 150 such as solder balls maybe then attached to the metalized vias 132. The conductive elements arethen preferably reflowed to permanent attach the conductive elements tothe metalized vias. During reflow, surface tension preferably reshapesthe outer surface of the conductive elements so that the conductiveelements have a substantially spherical shape as shown in FIG. 2H. Afterconductive elements 150 have been attached, the microelectronic packageof 2H may be electrically interconnected with another element via theconductive elements 150.

Referring to FIG. 2I, the microelectronic assembly of FIG. 2H may besevered to provide a plurality of microelectronic packages having anarray of resilient leads. As shown in FIG. 2I, semiconductor wafer 134,encapsulant layer 148 and flexible tape 120 are severed to providemicroelectronic packages 160A and 160B. Although only twomicroelectronic packages are shown in FIG. 2I, the wafer 134 may besevered to provide a plurality of microelectronic packages (e.g.,100-200 chip packages or more). Each microelectronic package desirableincludes at least one semiconductor chip 162, a portion of flexible tape120 and an array of resilient leads 146 that electrically interconnectchip 162 with conductive elements 150. As such, the microelectronicpackages 160A, 160B may be electrically interconnected with otherelements such as a test socket, a circuitized substrate or a printedcircuit board. During operation of the microelectronic packages 160A and160B, the various components will typically heat up. As the componentsheat up, the components may expand at different rates due to differencesin coefficients of thermal expansion. However, the resilient nature ofcomposite leads 146, encapsulant layer 148, and flexible tape 120 willallow the semiconductor chip 162 move relative to substrate 120 so as toremain electrically interconnected with conductive elements 150.

FIGS. 3A-3H show yet another preferred embodiment of a method of makinga microelectronic package having an array of resilient leads. Referringto FIG. 3A, a substrate 220, such as a two metal flexible tape, has afirst surface 222 and a second surface 224 remote therefrom. The twometal tape 220 includes a plurality of conductive leads 226 formedthereon. Each conductive leads 226 has a first end 230 releasablesecured to the first surface 222 of two metal tape 220 and a second orterminal end 228 permanently fixed to two metal tape 220. The terminalend 228 of conductive leads 226 overlie through vias 232, then throughvias 232 extending between the first and second surfaces 222, 224 of twometal tape 220.

Referring to FIG. 3B, a fixture such as a sacrificial layer may then bejuxtaposed with two metal tape 220. Fixture 234 includes contact bearingsurface 236 having a plurality of contacts 238 formed thereon and a backsurface 240 remote therefrom. Referring to FIG. 3C, fixture 234 may bejuxtaposed with two metal tape 220 so that contacts 238 are insubstantial alignment with the releasable ends 230 of leads 226.Contacts 230 are preferably permanently attached to releasable tip ends230 of conductive leads 226, such as by using a bonding process or aconductive adhesive.

Referring to FIG. 3D, in order to move fixture 234 and tape 220 awayfrom one another, platens 240 and 242 are preferably abutted againstfixture 234 and two metal tape 220, respectively. As described above,platens 240, 242 are used to controllably move fixture 234 and two metaltape 220 away from one another in a vertical direction. Fixture 234 andsubstrate 220 may also be moved relative to one another in a horizontaldirection. As fixture 234 and two metal tape 220 move away from oneanother, conductive leads 226 are extended in a substantially verticaldirection.

Referring to FIG. 3E, a layer of a spring-like material 244 may then bedeposited over the exterior surface of conductive leads 226 to formcomposite leads 246. As mentioned above, the formation of a layer of aspring-like material 244 over conductive leads 226 improves the overallresilience of the final structure, i.e., composite lead 246. Thisimproved resiliency enhances the ability of the lead to maintain anelectrical interconnection between microelectronic elements duringthermal cycling.

Referring to FIG. 3F, a layer of a curable liquid material 248 is thenpreferably deposited between fixture 234 and two metal tape 220 andaround composite leads 246. In preferred embodiments, the layer ofcurable material 248 may then be cured to provide a compliant materialthat enables the composite leads 246 to flex and bend during thermalcycling.

Referring to FIG. 3G, the fixture 234 may then be removed to transformthe subassembly into a connection component. In certain embodiments thefixture 234 is completely removed, such as by exposing the subassemblyto a chemical etchant. In other embodiments, the fixture may becomprised of a conductive material and may be provided without contacts238. Portions of the conductive fixture may then be removed. Theremaining portions form contacts in the tip ends of the leads. Afterfixture 234 has been removed, contacts 238 are exposed at a top surfaceof encapsulant layer 248. As mentioned above, the subassembly shown inFIG. 3G may be used as a compliant connection component 292 that canelectrical interconnect two or more microelectronic elements. In certainembodiments, the contacts of a first microelectronic element may beconnected with the contacts 238 exposed at a top surface of encapsulantlayer 248. In turn, contacts of a second microelectronic element may bepermanently or temporarily attached to terminals exposed at the secondsurface 224 of two metal tape 220.

In FIG. 3H, a test fixture 270 having conductive elements 272 at a topsurface thereof, is utilized to test the subassembly shown in FIG. 3G.The conductive elements 272 of the test fixture are preferably providedin a spaced array, the conductive elements 272 matching the alignment ofterminals 290 of connection component 292. After connection component292 has been positioned atop test fixture 270, a microelectronic elementor other electronic element having contacts may be juxtaposed with thecontacts 238 at the top of compliant layer 248, thereby allowing theconnection component to be tested and evaluated. Alternatively oradditionally, connection component 292 may be used to permanentlyconnect two microelectronic elements.

Referring to FIGS. 4 and 5, the leads shown and described above may bearranged in many different ways on wafers, flexible substrates, flexibletapes and other microelectronic elements. For example, referring to FIG.4, each lead 326 its initial undeformed state, may include an S-shapedstrip 380 extending between the terminal ends 328 and tip ends 330thereof. The S-shaped lead structures may be nested as shown in FIG. 4with the terminal ends 328 deposed in rows and the tip ends 330 deposedin similar but offset rows. Referring to FIG. 5, the leads 426 may alsobe substantially U-shaped structures having a single bight between theterminal end 428 and tip end 430 of each lead. Structures with pluralbights can also be employed. Such leads are shown and described incertain preferred embodiments of commonly assigned U.S. Pat. No.5,518,964, the disclosure of which is hereby incorporated by referenceherein.

The conductive leads may also have the various configurations shown inFIG. 6 and disclosed in the above-mentioned '964 patent, as well as incommonly assigned U.S. Pat. Nos. 5,859,472 and 6,191,368,the disclosuresof which are hereby incorporated by reference herein. As a result, anygap 586 surrounding the conductive leads may have correspondingly variedshapes. In each case, the gaps extend alongside the flexible, conductiveleads. Lead 526 is in the form of a closed loop 588 connecting thesecond end 530 of flexible lead with the first end 528 thereof. Theclosed loop section 588 of lead 526 encircles a central region 590.

Referring to FIG. 7, in still other preferred embodiments, restrainingstraps 692, which are shorter and stronger than conductive leads 626,are connected between two microelectronic elements. Restraining straps692 may be formed during the same process steps used to make theconductive leads. Such restraining straps are disclosed in commonlyassigned U.S. Pat. No. 5,976,913, the disclosure of which is herebyincorporated by reference herein. After leads 626 electricallyinterconnect two or more microelectronic elements, restraining straps692 limit movement of the microelectronic elements away from one anotherso that sufficient slack remains in the flexible, conductive leads 626.

Referring to FIG. 8, in yet further preferred embodiments, the tip end730 of each lead 726 is connected through a frangible element 794 to theterminal end 728 of the next adjacent lead. The frangible element 794thus retains each tip end 730 in position, adjacent a surface of asubstrate 720 or semiconductor wafer. Frangible element 794 may beformed as a continuation of a strip constituting the lead itself, withV-shaped notches extending in the strip from opposite sides thereof.During the assembly process, the tip ends 730 are bonded to the contactsof a chip or other microelectronic element in the same manner asdiscussed above. After bonding, the microelectronic element is movedrelative to the connector body or dielectric sheet in the same manner asdiscussed above, so that the tip end 730 of each lead 726 movesvertically away from the body and away from the terminal ends 728, andso that the tip end 720 also moves toward the associated terminal end728. This action breaks the frangible element 794 and hence, releaseseach tip end from its connection to the next terminal end. Such leadsare disclosed are certain preferred embodiments of the '964 patent.

Referring to FIG. 9, in still other preferred embodiments of the presentinvention, the tip ends 830 of each lead 826 is not provided with abulge, but instead constitutes a continuation of lead 826. The tip end830 of the lead is connected to the terminal end 828 of the nextadjacent lead by a frangible section 894. In this component, thedielectric sheet or connector body 820 has holes 832 aligned with theterminal ends 828 of the leads 826. After connector body 820 and theleads 826 thereon are in alignment with contacts on a microelectronicelement or chip, a tool (not shown) is advanced through holes 832 forengaging the tip ends 830 of each lead 826 in succession so as to bond atip ends 830 to contact. After such bonding, the microelectronicelements or chip may be moved relative to the connector body in the samemanner as discussed above. Once again, this movement breaks thefrangible section 894 between the tip end of each lead and terminal end828 of the adjacent lead, thus releasing the tip ends 830 and allowingthe leads to bend away from the connector body. Before or after themovement step, holes 832 may be closed by application of a further filmor sheet on the top surface of the dielectric layer.

Although the present invention has been described with reference toparticular preferred embodiments, it is to be understood that theembodiments are merely illustrative of the principle and application ofthe present invention. It is therefor to be understood that numerousmodifications may be made to the preferred embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the claims.

What is claimed is:
 1. A method of making a microelectronic packagehaving an array of resilient leads comprising: providing a first elementhaving a plurality of conductive leads at a first surface thereof, saidconductive leads having terminal ends permanently attached to said firstelement and tip ends remote from the terminal ends, the tip ends of saidconductive leads being movable relative to said terminal ends; providinga second element having a plurality of contacts on a first surfacethereof and juxtaposing the first surface of said second element withthe first surface of said first element; connecting the tip ends of saidconductive leads with the contacts of said second element; moving saidfirst and second elements away from one another so as to verticallyextend said conductive leads between said first and second elements; andafter the moving step, forming a layer of a spring-like material oversaid conductive leads.
 2. The method as claimed in claim 1, wherein saidconductive leads are made of a material selected from the groupconsisting of aluminum, gold, copper, tin, their alloys, andcombinations thereof.
 3. The method as claimed in claim 1, wherein saidlayer of spring-like material is selected from the group consisting ofnickel, copper, cobalt, iron, gold, silver, platinum, noble metals,semi-noble metals, tungsten, molybdenum, tin, leads, bismuth, indium,their alloys, and combinations thereof.
 4. The method as claimed inclaim 3, wherein said layer of a spring-like material is selected fromthe group consisting of nickel and nickel alloys.
 5. The method asclaimed in claim 1, wherein the layer of a spring-like material has agreater yield strength than said conductive leads.
 6. The method asclaimed in claim 1, further comprising depositing a curable liquidencapsulant between said first and second elements and around saidvertically extended leads.
 7. The method as claimed in claim 6, furthercomprising curing said encapsulant to provide a compliant layer betweensaid first and second elements.
 8. The method as claimed in claim 6,wherein said curable liquid encapsulant is selected from the groupconsisting of elastomers and adhesives.
 9. The method as claimed inclaim 6, wherein said curable liquid encapsulant is curable to asilicone elastomer.
 10. The method as claimed in claim 1, wherein saidterminals are accessible at the second surface of said second element.11. The method as claimed in claim 10, further comprising attachingconductive elements to the terminal ends of said leads.
 12. The methodas claimed in claim 10, wherein said conductive elements comprise solderballs.
 13. The method as claimed in claim 1, wherein said first andsecond elements are selected from the group consisting of asemiconductor chip, a semiconductor wafer, and a flexible circuitizedsubstrate.
 14. The method as claimed in claim 13, wherein said firstelement is a semiconductor chip or wafer and the second element is aflexible circuitized substrate.
 15. The method as claimed in claim 13,wherein said second element is a semiconductor chip or wafer and thefirst element is a flexible circuitized substrate.
 16. The method asclaimed in claim 1, wherein at least one of said first and secondelements is a sacrificial layer.
 17. The method as claimed in claim 16,further comprising removing at least a portion of said first element soas to expose the terminal ends of said conductive leads.
 18. The methodas claimed in claim 17, wherein the first element is a firstmicroelectronic element and the second element is a secondmicroelectronic element.
 19. The method as claimed in claim 16, furthercomprising removing at least a portion of said second element so as toexpose the contacts connected to the tip ends of said conductive leads.20. The method as claimed in claim 1, wherein said first element is afirst microelectronic element and said second element is a secondmicroelectronic element.
 21. A method of making a microelectronicpackage having a plurality of resilient leads comprising: providing afirst element having conductive leads extending along a first surfacethereof, said conductive leads having terminal ends permanently attachedto said first element and tip ends releasably secured to said firstelement; providing a second element having contacts on a first surfacethereof and juxtaposing the first surface of said second element withthe first surface of said first element; connecting the tip ends of saidconductive leads with the contacts of said second element; moving saidfirst and second elements away from one another so as to verticallyextend said conductive leads between said first and second elements; andafter the moving step, forming a layer of a spring-like material oversaid conductive leads.
 22. The method as claimed in claim 21, whereinthe forming a layer of a spring-like material step includes plating aconductive metal over said conductive leads.
 23. The method as claimedin claim 21, wherein said layer of a spring-like material has a greateryield strength than said conductive leads.
 24. The method as claimed inclaim 21, wherein the first element is a first microelectronic elementand the second element is a second microelectronic element.
 25. Themethod as claimed in claim 21, further comprising: disposing a curableliquid encapsulant between said first and second microelectronicelements and around said conductive leads; and curing said curableliquid encapsulant to form a compliant layer between said first andsecond microelectronic elements.
 26. The method as claimed in claim 25,wherein said encapsulant is selected from the group consisting onelastomers and adhesives.
 27. The method as claimed in claim 21, whereinsaid first microelectronic element is a flexible substrate and saidsecond microelectronic element includes a semiconductor chip.
 28. Themethod as claimed in claim 21, further comprising attaching conductiveelements to the terminal ends of said leads, wherein said conductiveelements are accessible at a second surface of said first element. 29.The method as claimed in claim 21, wherein the terminal ends of saidconductive leads extend between the first surface and a second surfaceof said first microelectronic element.
 30. A method of making amicroelectronic package comprising: providing a first element having afirst surface with a plurality of conductive leads formed thereon, eachsaid lead having a first end permanently attached to said first elementand a second end movable away from said first element; providing asecond element having conductive pads accessible at a first surfacethereof and juxtaposing the first surface of said first element with thefirst surface of said second element; attaching the tip ends of saidconductive leads with said conductive pads of said second element; afterthe attaching step, moving said first and second elements away from oneanother so as to vertically extend said conductive leads; and after themoving step, forming a layer of a conductive metal over said conductiveleads, wherein said layer of a conductive metal has a greater yieldstrength than said conductive leads.
 31. The method as claimed in claim30, wherein the attaching said tip ends step includes bonding the tipends of said leads to said conductive pads so as to electricallyinterconnect said leads and said conductive pads.
 32. The method asclaimed in claim 30, further comprising after the forming a layer of aconductive material step, providing a curable liquid encapsulant betweensaid microelectronic elements and around said conductive leads andcuring said encapsulant to provide a compliant layer.
 33. The method asclaimed in claim 32, wherein said first microelectronic element is asemiconductor wafer and said second microelectronic element is aflexible dielectric sheet.
 34. The method as claimed in claim 33,further comprising severing said semiconductor wafer and said flexibledielectric sheet to provide a plurality of semiconductor packages, eachsaid package including at least one semiconductor chip.
 35. The methodas claimed in claim 33, wherein the conductive pads of said secondmicroelectronic element are accessible at the first surface and a secondsurface of said second microelectronic element, the method furthercomprising attaching conductive elements to the conductive pads of saidsecond microelectronic element, said conductive elements overlying thesecond surface of said second microelectronic element.
 36. A method ofmaking semiconductor packages having resilient leads comprising:providing a first microelectronic element selected from the groupconsisting of semiconductor chips and semiconductor wafers, wherein saidfirst microelectronic element has a plurality of contacts on a firstsurface thereof; providing a flexible dielectric sheet having aplurality of conductive leads over a first surface thereof, each saidlead having a terminal end permanently attached to said flexibledielectric sheet and a tip end movable away from the first surface ofsaid dielectric sheet; electrically interconnecting the tip ends of saidleads to the contacts of said first microelectronic element; moving saidfirst microelectronic element and dielectric sheet away from one anotherso as to vertically extend said leads; after the moving step, forming alayer of a spring-like material over said leads, wherein the spring-likematerial has a greater yield strength than the conductive leads.
 37. Themethod as claimed in claim 36, further comprising providing a layer of acompliant material between said first microelectronic element and saiddielectric sheet and around said leads.
 38. The method as claimed inclaim 37, further comprising after the providing a layer of a compliantmaterial step, severing said first microelectronic element and saiddielectric sheet to provide a plurality of semiconductor packages, eachsaid semiconductor package comprising at least one semiconductor chipand a portion of said dielectric sheet.
 39. A method of making amicroelectronic element comprising: providing a dielectric sheet havinga plurality of conductive leads overlying a first surface of said sheetand a plurality of terminals accessible at a second surface of saidsheet, each said lead having a first end permanently attached to one ofsaid terminals and a second end movable away from the first surface ofsaid dielectric sheet; providing a fixture having a first surface and aplurality of contacts accessible at the first surface of said fixture;juxtaposing the first surface of said fixture with the first surface ofsaid dielectric sheet and attaching the second ends of said leads withthe contacts of said fixture; after the attaching step, moving saidfixture and said dielectric sheet away from one another so as tovertically extend said leads; forming a layer of a conductivespring-like material over said leads; providing a layer of a curableliquid encapsulant between said fixture and said dielectric sheet andaround said leads and curing said encapsulant to form a compliant layer;and after the curing step, removing said fixture so as to expose saidcontacts at a top surface of said package.
 40. The method as claimed inclaim 39, further comprising temporarily connecting said contacts at thetop surface of said package with the contacts of an element.
 41. Themethod as claimed in claim 40, wherein said microelectronic element is atest fixture.
 42. The method as claimed in claim 39, further comprisingtemporarily connecting the terminals of said dielectric sheet with thecontacts of an element.
 43. The method as claimed in claim 42, whereinsaid microelectronic element is a test fixture.